utf8

package standard library
go1.25.3 Latest Latest
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 Go to latest Published: Oct 13, 2025 License: BSD-3-Clause Imports: 0 Imported by: 126,051

Documentation

Overview

Package utf8 implements functions and constants to support text encoded in UTF-8. It includes functions to translate between runes and UTF-8 byte sequences. See https://en.wikipedia.org/wiki/UTF-8

Index

Examples

Constants

View Source 
const (
	RuneError = '\uFFFD'     // the "error" Rune or "Unicode replacement character"
	RuneSelf  = 0x80         // characters below RuneSelf are represented as themselves in a single byte.
	MaxRune   = '\U0010FFFF' // Maximum valid Unicode code point.
	UTFMax    = 4            // maximum number of bytes of a UTF-8 encoded Unicode character.
)

Numbers fundamental to the encoding.

Variables

This section is empty.

Functions

func AppendRune added in go1.18 

func AppendRune(p []byte, r rune) []byte

AppendRune appends the UTF-8 encoding of r to the end of p and returns the extended buffer. If the rune is out of range, it appends the encoding of RuneError.

Example 
package main

import (
	"fmt"
	"unicode/utf8"
)

func main() {
	buf1 := utf8.AppendRune(nil, 0x10000)
	buf2 := utf8.AppendRune([]byte("init"), 0x10000)
	fmt.Println(string(buf1))
	fmt.Println(string(buf2))
}

Output:

𐀀
init𐀀

func DecodeLastRune 

func DecodeLastRune(p []byte) (r rune, size int)

DecodeLastRune unpacks the last UTF-8 encoding in p and returns the rune and its width in bytes. If p is empty it returns (RuneError, 0). Otherwise, if the encoding is invalid, it returns (RuneError, 1). Both are impossible results for correct, non-empty UTF-8.

An encoding is invalid if it is incorrect UTF-8, encodes a rune that is out of range, or is not the shortest possible UTF-8 encoding for the value. No other validation is performed.

Example 
package main

import (
	"fmt"
	"unicode/utf8"
)

func main() {
	b := []byte("Hello, 世界")

	for len(b) > 0 {
		r, size := utf8.DecodeLastRune(b)
		fmt.Printf("%c %v\n", r, size)

		b = b[:len(b)-size]
	}
}

Output:

界 3
世 3
  1
, 1
o 1
l 1
l 1
e 1
H 1

func DecodeLastRuneInString 

func DecodeLastRuneInString(s string) (r rune, size int)

DecodeLastRuneInString is like DecodeLastRune but its input is a string. If s is empty it returns (RuneError, 0). Otherwise, if the encoding is invalid, it returns (RuneError, 1). Both are impossible results for correct, non-empty UTF-8.

An encoding is invalid if it is incorrect UTF-8, encodes a rune that is out of range, or is not the shortest possible UTF-8 encoding for the value. No other validation is performed.

Example 
package main

import (
	"fmt"
	"unicode/utf8"
)

func main() {
	str := "Hello, 世界"

	for len(str) > 0 {
		r, size := utf8.DecodeLastRuneInString(str)
		fmt.Printf("%c %v\n", r, size)

		str = str[:len(str)-size]
	}
}

Output:

界 3
世 3
  1
, 1
o 1
l 1
l 1
e 1
H 1

func DecodeRune 

func DecodeRune(p []byte) (r rune, size int)

DecodeRune unpacks the first UTF-8 encoding in p and returns the rune and its width in bytes. If p is empty it returns (RuneError, 0). Otherwise, if the encoding is invalid, it returns (RuneError, 1). Both are impossible results for correct, non-empty UTF-8.

An encoding is invalid if it is incorrect UTF-8, encodes a rune that is out of range, or is not the shortest possible UTF-8 encoding for the value. No other validation is performed.

Example 
package main

import (
	"fmt"
	"unicode/utf8"
)

func main() {
	b := []byte("Hello, 世界")

	for len(b) > 0 {
		r, size := utf8.DecodeRune(b)
		fmt.Printf("%c %v\n", r, size)

		b = b[size:]
	}
}

Output:

H 1
e 1
l 1
l 1
o 1
, 1
  1
世 3
界 3

func DecodeRuneInString 

func DecodeRuneInString(s string) (r rune, size int)

DecodeRuneInString is like DecodeRune but its input is a string. If s is empty it returns (RuneError, 0). Otherwise, if the encoding is invalid, it returns (RuneError, 1). Both are impossible results for correct, non-empty UTF-8.

An encoding is invalid if it is incorrect UTF-8, encodes a rune that is out of range, or is not the shortest possible UTF-8 encoding for the value. No other validation is performed.

Example 
package main

import (
	"fmt"
	"unicode/utf8"
)

func main() {
	str := "Hello, 世界"

	for len(str) > 0 {
		r, size := utf8.DecodeRuneInString(str)
		fmt.Printf("%c %v\n", r, size)

		str = str[size:]
	}
}

Output:

H 1
e 1
l 1
l 1
o 1
, 1
  1
世 3
界 3

func EncodeRune 

func EncodeRune(p []byte, r rune) int

EncodeRune writes into p (which must be large enough) the UTF-8 encoding of the rune. If the rune is out of range, it writes the encoding of RuneError. It returns the number of bytes written.

Example 
package main

import (
	"fmt"
	"unicode/utf8"
)

func main() {
	r := '世'
	buf := make([]byte, 3)

	n := utf8.EncodeRune(buf, r)

	fmt.Println(buf)
	fmt.Println(n)
}

Output:

[228 184 150]
3
Example (OutOfRange) 
package main

import (
	"fmt"
	"unicode/utf8"
)

func main() {
	runes := []rune{
		// Less than 0, out of range.
		-1,
		// Greater than 0x10FFFF, out of range.
		0x110000,
		// The Unicode replacement character.
		utf8.RuneError,
	}
	for i, c := range runes {
		buf := make([]byte, 3)
		size := utf8.EncodeRune(buf, c)
		fmt.Printf("%d: %d %[2]s %d\n", i, buf, size)
	}
}

Output:

0: [239 191 189] � 3
1: [239 191 189] � 3
2: [239 191 189] � 3

func FullRune 

func FullRune(p []byte) bool

FullRune reports whether the bytes in p begin with a full UTF-8 encoding of a rune. An invalid encoding is considered a full Rune since it will convert as a width-1 error rune.

Example 
package main

import (
	"fmt"
	"unicode/utf8"
)

func main() {
	buf := []byte{228, 184, 150} // 世
	fmt.Println(utf8.FullRune(buf))
	fmt.Println(utf8.FullRune(buf[:2]))
}

Output:

true
false

func FullRuneInString 

func FullRuneInString(s string) bool

FullRuneInString is like FullRune but its input is a string.

Example 
package main

import (
	"fmt"
	"unicode/utf8"
)

func main() {
	str := "世"
	fmt.Println(utf8.FullRuneInString(str))
	fmt.Println(utf8.FullRuneInString(str[:2]))
}

Output:

true
false

func RuneCount 

func RuneCount(p []byte) int

RuneCount returns the number of runes in p. Erroneous and short encodings are treated as single runes of width 1 byte.

Example 
package main

import (
	"fmt"
	"unicode/utf8"
)

func main() {
	buf := []byte("Hello, 世界")
	fmt.Println("bytes =", len(buf))
	fmt.Println("runes =", utf8.RuneCount(buf))
}

Output:

bytes = 13
runes = 9

func RuneCountInString 

func RuneCountInString(s string) (n int)

RuneCountInString is like RuneCount but its input is a string.

Example 
package main

import (
	"fmt"
	"unicode/utf8"
)

func main() {
	str := "Hello, 世界"
	fmt.Println("bytes =", len(str))
	fmt.Println("runes =", utf8.RuneCountInString(str))
}

Output:

bytes = 13
runes = 9

func RuneLen 

func RuneLen(r rune) int

RuneLen returns the number of bytes in the UTF-8 encoding of the rune. It returns -1 if the rune is not a valid value to encode in UTF-8.

Example 
package main

import (
	"fmt"
	"unicode/utf8"
)

func main() {
	fmt.Println(utf8.RuneLen('a'))
	fmt.Println(utf8.RuneLen('界'))
}

Output:

1
3

func RuneStart 

func RuneStart(b byte) bool

RuneStart reports whether the byte could be the first byte of an encoded, possibly invalid rune. Second and subsequent bytes always have the top two bits set to 10.

Example 
package main

import (
	"fmt"
	"unicode/utf8"
)

func main() {
	buf := []byte("a界")
	fmt.Println(utf8.RuneStart(buf[0]))
	fmt.Println(utf8.RuneStart(buf[1]))
	fmt.Println(utf8.RuneStart(buf[2]))
}

Output:

true
true
false

func Valid 

func Valid(p []byte) bool

Valid reports whether p consists entirely of valid UTF-8-encoded runes.

Example 
package main

import (
	"fmt"
	"unicode/utf8"
)

func main() {
	valid := []byte("Hello, 世界")
	invalid := []byte{0xff, 0xfe, 0xfd}

	fmt.Println(utf8.Valid(valid))
	fmt.Println(utf8.Valid(invalid))
}

Output:

true
false

func ValidRune added in go1.1 

func ValidRune(r rune) bool

ValidRune reports whether r can be legally encoded as UTF-8. Code points that are out of range or a surrogate half are illegal.

Example 
package main

import (
	"fmt"
	"unicode/utf8"
)

func main() {
	valid := 'a'
	invalid := rune(0xfffffff)

	fmt.Println(utf8.ValidRune(valid))
	fmt.Println(utf8.ValidRune(invalid))
}

Output:

true
false

func ValidString 

func ValidString(s string) bool

ValidString reports whether s consists entirely of valid UTF-8-encoded runes.

Example 
package main

import (
	"fmt"
	"unicode/utf8"
)

func main() {
	valid := "Hello, 世界"
	invalid := string([]byte{0xff, 0xfe, 0xfd})

	fmt.Println(utf8.ValidString(valid))
	fmt.Println(utf8.ValidString(invalid))
}

Output:

true
false

Types

This section is empty.

Source Files

View all Source files

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